As a silicone rubber composition capable of curing by heating in a short time, there are a peroxide vulcanization type silicone rubber which forms a rubbery elastic body by crosslinking by compounding a silicone raw rubber comprising a straight chain highly polymerized polyorganosiloxane with an organic peroxide followed by heating and an addition reaction type (platinum vulcanization type) silicone rubber which forms a rubbery elastic body by carrying out crosslinking by the addition reaction between a vinyl group-containing polyorganosiloxane and a polyorganohydrogensiloxane in the presence of a platinum catalyst, and these silicone rubbers are widely used. In the former, it is required to complete the crosslinking by secondary heating called a postcure after carrying out the primary molding; and in the latter, in the case of moldings which are used in a place to which a compressive stress is applied for a long time, the postcure is necessary to particularly reduce the compression set.
Injection molding is frequently employed for the mass production of moldings. Thus, for generally rationalizing the production steps of moldings, a silicone rubber composition which can provide moldings having a small compression set without the need of a postcure step has been desired. This need has become recently particularly remarkable with the increase of the use of integral injection molded products (composite moldings) with various kinds of thermoplastic resins which cannot endure the postcuring temperature of an ordinary silicone rubber.
For filling such a need, investigations have been made on both the base polymer and various kinds of compounding agents. As the former, there is, for example, a method of bonding plural vinyl groups to each of both terminals of the molecular chain of the base polymer as disclosed in JP-A-63-270763 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and as the latter, there are, for example, a method of adding a triazole compound as disclosed in JP-A-2-242854 and a method of adding a vinyl group-containing organosilazane as disclosed in JP-A-63-268764. However, in the former method, the synthesis of the base polymer is complicated which increases the cost and in the latter methods, there are problems in the points of safety and hygiene and also sufficient results are not obtained.
On the other hand, it has been widely desired to impart a flame retardance to injection molded products. It is known that when a flame retardance imparting agent such as a platinum compound, carbon black, an azo compound, a triazole compound, iron oxide, etc., is compounded with the above-described peroxide vulcanization type silicone rubber compositions, the flame retardance thereof is improved. It is also known that the flame retardance of these silicone rubber compositions is more improved by increasing the compounded amount of the filler which is an incombustible component and reducing the compounded amount of a polyorganosiloxane which is a combustible component. As such a flame retarding technique, a method for rendering a silicone rubber flame retardant by using a carboxylic acid amide and cerium oxide together is disclosed in JP-B-60-1895 (the term "JP-B" as used herein means an "examined published Japanese patent application"). However, since such a technique is by a crosslinking mechanism of a peroxide vulcanization, it is necessary to carry out a postcure and also in the method, even when a postcure is carried out, a good effect is not obtained about the compression set. Furthermore, in the flame retardant silicone rubber compositions obtained by this method, the apparent viscosities of the uncured mixtures are all very high, and the silicone compositions cannot be used for use requiring flowability, such as a material for injection molding.
On the other hand, it has been attempted to improve the flame retardance of the addition reaction type silicone rubber composition using a flowable polyorganosiloxane as the base polymer by compounding therewith the flame retardance imparting agent as described above. However, in this method, for obtaining a satisfactory flame retardance and a satisfactory mechanical strength, it is required to extremely increase the compounding amount of a reinforcing filler and also if the flame retardance is improved, the compression set is deteriorated, whereby it is difficult to simultaneously satisfy both of these characteristics, which has been desired.